Code Division Multiple Access (CDMA) is a now common method for transmission of voice and data over radio. TruePosition was a pioneer in location of CDMA mobiles when in the year 2000, it conducted extensive testing with Verizon Wireless in mid-town Manhattan, N.Y. Verizon Laboratories used the rigorous test plan published by the CDMA Development Group (CDG) to determine the performance of TruePosition's network-based location technology in the challenging urban canyon (10 to 25 story buildings) environment. The WLS demonstrated sub-100 meter location results in a variety of indoor, outdoor, pedestrian, and moving vehicle scenarios. In the trial, unmodified CDMA (IS-95) mobile phones were used to make more than 30,000 test calls. These calls were placed by both Verizon Labs (formerly GTE Labs) and TruePosition in an area covered by 30 cell sites hosting time difference of arrival (TDOA) receivers.
The present invention derives in part from the data collected during the 2000 trial and is a distinct improvement over the trial system in terms of location performance. The disclosed techniques are also useful in a hybrid solution where assisted GPS (A-GPS) and Uplink Time Difference of Arrival (U-TDOA) technologies operate independently and simultaneously to obtain range estimates that can be combined in a final TDOA calculation or operate in fallback mode where U-TDOA is used when A-GPS fails. Use of a hybrid wireless location system, using the inventive concepts, creates an improved location solution with enhanced accuracy, yield, and performance.
The inventive techniques and concepts described herein apply to code-division radio communications systems, including the technologies referred to in technical specifications as CDMAOne (TIA/EIA IS-95 CDMA with IS-95A and IS-95B revisions), CDMA2000 family of radio protocols (as defined by the 3rd Generation Partnership Project 2 (3GPP2)), and in the Wideband Code-Division Multiple-Access (W-CDMA) radio system defined by the 3rd Generation Partnership Project (3GPP) as part of the Universal Mobile Telephone System (UMTS). The UMTS model discussed herein is an exemplary but not exclusive environment in which the present invention may be used.
The needs of a CDMA-based wireless communications system and a wireless location system (WLS) differ. In a CDMA-based communications system, due to the multi-path radio environment, multiple time delayed, frequency offset copies of the transmitted signal arrive at the receiver antenna. Due to the design of the CDMA codes and receiver, these delayed waveforms can be recovered and combined to yield an improved signal since the direct wave is not necessarily the best signal.
Using CDMA, both the base station and mobile receivers use rake receiver techniques first devised by Price and Green in 1958. (See Price, R.; Green, P. Jr.; “A Communication Technique for Multipath Channels,” Proc. IRE, vol. 46, pp. 555-570, March 1958.) Each correlator in a rake receiver is called a “finger”. The outputs of rake-receiver fingers may be combined coherently or non-coherently. Typical CDMA receivers use 3 to 5 fingers. There are two primary real-time methods used to combine the rake-receiver finger outputs and thus boost the signal gain. These signal amplification techniques are equal-gain combining or maximal-ratio combining. These signal amplification techniques are useful for wireless communications, but since the timing of the CDMA signal is perturbed by the combinational techniques, they are less useful for TDOA-based wireless location.
A WLS may use the well-known basic CDMA rake receiver design, but due to the non-real time constraints of a WLS versus a voice/data wireless communications system, other signal collection and signal processing techniques provide better processing gain than the standard bitwise equal-gain combining or maximal-ratio combining while preserving the minimally time delayed signal for determination of TDOA timing. As described herein, a combination of processing techniques may be used to determine the valid minimally time delayed signal in the presence of time-delayed, frequency offset multipath CDMA signals for the purpose of U-TDOA location.